Fermented milk is a paste or liquid product obtained by fermenting milk, or a milk product, etc. containing solid contents of nonfat milk at the same level or higher than milk, with lactic acid bacteria or yeast, or a frozen product thereof, and is broadly classified into two types. One is a pre-fermentation type, and the other is a post-fermentation type. The former is produced by packing into an individual container for distribution the fermented milk crushed after the completion of fermentation and cooling in a tank before packing into the container. The latter is produced by packing a raw material mix for fermented milk with the addition of a predetermined amount of a starter (hereinafter referred to as a mix) into an individual container for distribution such as a paper container, and subsequently fermenting the mix in a fermentation chamber until the lactic acid acidity reaches a predetermined level for solidification into a pudding-like state, followed by cooling. The pre-fermentation is commonly used for producing fruit yogurt with fruit flesh, drinking yogurt and the like, while the post-fermentation is commonly used for producing yogurt of so-called hard-type and plain-type, and the like.
In either case of the fermentation types, a starter is added to a sterilized mix at a predetermined temperature, and fermentation is carried out until the acidity reaches a predetermined level in the fermentation process. Then, the resulting product is cooled for terminating the fermentation to prepare a final product. Because the fermentation temperature and fermentation time at this step not only affect the production efficiency of the product but also significantly affect the taste and quality, it is necessary to set the fermentation temperature and the fermentation time appropriately with taking those effects into account. For example, the fermentation temperature should be set lower in the case where it is intended to give a mild taste to the product.
What is problematic here is that, for example in the case where the fermentation temperature is set lower to give a mild taste to the product, problems arise in the production efficiency and product quality, for example, the fermentation time required to reach a predetermined acidity is significantly prolonged compared with a usual method, the texture hardness is so low that the product collapses during the distribution process and so on. Accordingly, the present situation is that it is difficult to obtain a fermented milk having a mild taste and a texture with a hardness that does not collapse during the distribution process without prolonging the fermentation time.
No direct method for improving such a present situation has been reported yet. However, the following approaches have been conventionally carried out as applicable methods. A first approach is a method of adjusting conditions in which the efficiency of fermentation process is improved to shorten the fermentation time. It is considered that such an approach enables fermentation at a fermentation temperature lower than usual in addition to the achievement of the object of improving the production efficiency. A second approach is a method of selecting a lactic acid bacterium which produces a substance giving a mild taste. A third approach includes a method of not devising the fermentation process but giving a necessary texture such as mildness by adding an additive for improving the taste to the product.
As the first method for accelerating fermentation, a method of adding a milk protein concentrate (JP-A-11-028056), a method of adding a butter milk (JP-A-09-201164) and the like have been proposed. However, in these prior examples, low-temperature fermentation was not assumed, and it was not mentioned whether or not fermentation can be accelerated at a low temperature.
As the second method, a method of giving a fresh and mild taste to a fermented milk by fermentation in which a lactic acid bacterium to be used is selected so as to make L-lactic acid account for 85% or more of the total lactic acid (JP-A-06-327401) has been proposed.
As the third method, a method of improving the taste by adding an amino acid having a specific composition to a resulting fermented milk (for example, JP-A-10-327751) has been proposed.
Any of these methods involves the addition of some substance to a fermented milk or the modification of the composition of the components thereof. Therefore, it is inevitable to design the product in consideration of the effects of additives and the like on the product's taste, quality and the like. In other words, these methods can neither improve production efficiency while maintaining the product feature of the original product without using additives and the like, nor add a “mild taste” to the product while maintaining the production efficiency and the product feature of the original product.
As the method of giving the fermented milk a hardness that does not allow the texture to collapse during transportation or the like, there is a report describing that because the texture softens when a fermented milk is produced by adding peroxidase, the texture can be prevented from softening by reducing the dissolved oxygen concentration (JP-A-10-099019).
In this case, the description on fermentation temperature including the case of generally performing fermentation at a low temperature can be found, however, there is no suggestion that fermentation time is shortened when dissolved oxygen is reduced or a fermented milk with a smooth texture and an unprecedented taste can be obtained by carrying out fermentation at a lower fermentation temperature for a shorter time compared with the case where the dissolved oxygen concentration is not reduced. As a matter of course, the invention in this case relates to a method for improving a fermented milk to which peroxidase was added, and does not relate to a fermented milk in which an additive that affects the texture is not contained as the case of obtaining a hard yogurt of the method of the present invention.